1. Field of the Invention
The present invention relates to methods for performing thin film layer thickness metrology and, more particularly, to an apparatus and method for performing thin film layer thickness metrology on a silicon/silicon dioxide/silicon (Si/SiO.sub.2 /Si) structured semiconductor wafer using a correlation reflectometer.
2. Description of the Prior Art
In one particular application wherein the present invention is especially practical, a silicon-on-insulator (SOI) semiconductor wafer, typically consisting of an Si/SiO.sub.2 /Si sandwich structure, is fabricated by growing a silicon dioxide (SiO.sub.2) film on one surface of each of two silicon (Si) wafers and bonding the two silicon dioxide film surfaces together at high temperature. The SOI wafer then undergoes a further fabrication process, wherein an outer surface of one of the two silicon wafers in the sandwich structure is mechanically ground and polished to an avenge thickness of several microns. This mechanical grinding and polishing unfortunately results in large spatial variations in the thickness of this one silicon wafer, or this one outer silicon layer. To reduce these spatial variations, a thickness map that indicates thickness non-uniformities in this outer silicon layer over the entire wafer surface is required, for example, to initialize a subsequent micropolishing process.
A thickness map is obtained by measuring the spatial variations in the thickness of the outer silicon layer over the entire outer silicon layer surface. A sequence of measuring these spatial variations followed by thinning and smoothing the surface of this outer silicon layer by micropolishing may need to be performed several times before the entire outer silicon layer achieves a desired thickness. Due to the precision of a typical micropolishing instrument and in order to reduce costs and increase wafer production, a thickness measurement of at least 400 points on the outer silicon layer surface in 60 seconds or less is desirable.
Current commercial instruments can typically provide a thickness measurement of a thin film layer at a single point on the surface of the thin film layer. These instruments use a focused lens or a fiber bundle to locally illuminate the thin film layer surface with a beam of monochromatic light, and a grating or prism spectrograph to measure the surface spectral reflectance at each point. In all cases, this surface spectral reflectance data must be numerically corrected due to variations in the angle of incidence caused by the f-number of the illuminating beam. Also, the time required for these instruments to determine the thickness of an outer silicon layer of an SOI semiconductor wafer at a single point thereon is on the order of several minutes, which far exceeds the time desired for efficient wafer production.
These current commercial instruments can be extended to cover an entire thin film layer, such as an outer silicon layer of an SOI semiconductor wafer, by moving either the measuring instrument or the wafer in a controlled manner. However, as when measuring only a single point, the time required for these instruments to characterize an entire outer silicon layer with at least 400 measurement points far exceeds the time desired for efficient wafer production.
A recent advance in the state of the art is described in U.S. patent application Ser. No. 07/804,872, entitled, Apparatus and Method for Measuring the Thickness of Thin films, filed on Dec. 6, 1991, now pending and assigned to the assignee hereof. The above-referenced document describes an instrument that can perform a thin film layer thickness measurement over an entire thin film layer, such as an outer silicon layer of an SOI semiconductor wafer, in a time that is desirable for efficient wafer production. However, a disadvantage of this instrument, as well as with all of the previously described current commercial instruments, is a need for stored calibration or reference spectral data that is compared with measured spectral data so as to derive a "best fit" thin film layer thickness. This stored calibration or reference spectral data is obtained by assuming that the complex index of refraction of the thin film layer to be measured is known over the spectral region of analysis, and therefore the spectral data is subject to error. It is therefore desirable to perform thin film layer thickness metrology over an entire thin film layer, such as an entire outer silicon layer of an SOI semiconductor wafer, in a manner that is efficient, economical, and highly accurate, but does not require the use of stored calibration or reference spectral data.